/*****************************************************************************
* FILE: 10-mutex.c
* DESCRIPTION:
*   This example program illustrates the use of mutex variables 
*   in a threads program. This version was obtained by modifying the
*   serial version of the program (09-mutex.c) which performs a 
*   dot product. The main data is made available to all threads through 
*   a globally accessible  structure. Each thread works on a different 
*   part of the data. The main thread waits for all the threads to complete 
*   their computations, and then it prints the resulting sum.
*
*   An alternative to PTHREAD_MUTEX_INITIALIZER is used in this example
*
* SOURCE: Vijay Sonnad, IBM
* LAST REVISED: 01/29/09 Blaise Barney
******************************************************************************
  pthread.h should be first otherwise compile with -D_THREAD_SAFE 
*/
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>

/*   
  The following structure contains the necessary information  
  to allow the function "dotprod" to access its input data and 
  place its output into the structure.  This structure is 
  unchanged from the sequential version.
*/

typedef struct {
   double *a, *b, sum; 
   int veclen; 
 } DOTDATA;

/* Define globally accessible variables and a mutex */

#define NUMTHRDS 4
#define VECLEN 25000

/* shared structure */
DOTDATA dotstr; 
pthread_t callThd[NUMTHRDS];
pthread_mutex_t mutexsum;

/*
  The function dotprod is activated when the thread is created.
  As before, all input to this routine is obtained from a structure 
  of type DOTDATA and all output from this function is written into
  this structure. The benefit of this approach is apparent for the 
  multi-threaded program: when a thread is created we pass a single
  argument to the activated function - typically this argument
  is a thread number. All  the other information required by the 
  function is accessed from the globally accessible structure. 
*/

void *dotprod(void *arg) {

   /* Define and use local variables for convenience */
   int i, start, end, len ;
   long offset;
   double mysum, *x, *y;
   offset = (long)arg;
     
   len = dotstr.veclen;
   start = offset*len;
   end   = start + len;
   x = dotstr.a;
   y = dotstr.b;
   
   /*
     Perform the dot product and assign result
     to the appropriate variable in the structure. 
   */
   mysum = 0;
   for (i=start ; i < end ; i++) mysum += (x[i] * y[i]);
   
   /*
     Lock a mutex prior to updating the value in the shared
     structure, and unlock it upon updating.
   */

   pthread_mutex_lock (&mutexsum);
   dotstr.sum += mysum;
   printf("Thread %ld did %d to %d:  mysum=%f global sum=%f\n",
	  offset, start, end, mysum, dotstr.sum);
   pthread_mutex_unlock (&mutexsum);
   pthread_exit((void*)0);
}

/* 
   The main program creates threads which do all the work and then 
   print out result upon completion. Before creating the threads,
   The input data is created. Since all threads update a shared structure, we
   need a mutex for mutual exclusion. The main thread needs to wait for
   all threads to complete, it waits for each one of the threads. We specify
   a thread attribute value that allow the main thread to join with the
   threads it creates. Note also that we free up handles  when they are
   no longer needed.
*/

int main (int argc, char *argv[]) {
   long i;
   double *a, *b;
   void *status;
   pthread_attr_t attr;

   /* Assign storage and initialize values */
   a = (double*) malloc (NUMTHRDS*VECLEN*sizeof(double));
   b = (double*) malloc (NUMTHRDS*VECLEN*sizeof(double));
  
   for (i=0 ; i < VECLEN*NUMTHRDS ; i++) a[i] = b[i] = 1;

   dotstr.veclen = VECLEN; 
   dotstr.a = a; 
   dotstr.b = b; 
   dotstr.sum=0;
   
   /* alternative to using PTHREAD_MUTEX_INITIALIZER */
   pthread_mutex_init(&mutexsum, NULL);
         
   /* Create threads to perform the dotproduct  */
   pthread_attr_init(&attr);
   pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
   
   for (i=0 ; i < NUMTHRDS ; i++) {
      /* Each thread works on a different set of data.
       * The offset is specified by 'i'. The size of
       * the data for each thread is indicated by VECLEN.
       */
      pthread_create(&callThd[i], &attr, dotprod, (void *)i); 
   }

   pthread_attr_destroy(&attr);
   /* Wait on the other threads */

   for (i=0 ; i < NUMTHRDS ; i++) pthread_join(callThd[i], &status);

   /* After joining, print out the results and cleanup */
   printf ("Sum =  %f \n", dotstr.sum);
   free (a);
   free (b);
   pthread_mutex_destroy(&mutexsum);
   pthread_exit(NULL);
}   
